Electrical equipment and electrical installations remaining in operation over an extended period of time, such as pole transformers, have to be monitored and maintained in order to avoid accidents due to failure. The accidents and failures that are a concern include malfunctions related to power outages caused by a drop in voltage over the power network, fires, and service life. Existing electrical equipment has to be replaced with new equipment before the end of its service life. However, much electrical equipment is able to remain in service and it is uneconomical to replace the equipment on a regular schedule.